Thermochromic dyes are known for use in various applications including electrical systems applications. For example, thermochromic dyes have been used as a voltmeter in the prior art to determine the signal strength of batteries. In such an application, a thermochromic dye containing layer may be arranged in contact with an electrically conductive layer which extends between positive and negative electrodes of a battery. A switch would be used to open and close a current flow path so that when the switch is in a closed position, current from the battery is permitted to flow between the positive and negative electrodes through the conductive layer. Heat is generated as the current flows through the conductive layer thus activating the thermochromic dye in the adjacent layer so that a color change is obtained. Such a prior art system has shortcomings as it requires entirely separate layers of thermochromic ink and conductive materials. The separate layers contribute to increased manufacturing costs and printing steps. Further, problems may arise due to separation of the various layers. Examples of such prior art systems are disclosed in U.S. Pat. Nos. 5,059,895 to Capaldi et al.; 5,015,544 to Burroughs et al.; 4,835,476 to Kurosawa; and 4,835,475 to Hanakura et al.
The prior art also discloses the use of thermochromic materials for coating color changing designs onto substrates. For example, U.S. Pat. No. 4,028,118 to Nakasuji et al. teaches that color changing thermochromic compositions may be applied to the surface of a circuit board or machine and activated by heat generation caused by overloads in the electrical circuits or electrical machines to provide a warning of an overload condition. It is also disclosed that color changes could be used to hide or reveal a background for a display, exhibition, advertisement, teaching material, toy, magnetic device, or the like. As with the other prior art patents discussed above, the '118 patent requires a separate thermochromic layer in contact with a conductive material. All of the foregoing prior art systems have the shortcoming of requiring thermochromic compositions to be used in conjunction with at least one additional separate layer of conductive material. The present invention overcomes this shortcoming.